Electron-emitting material



Patented Mar. 27, 1928.

umrao STATES CHARLES V.

IREDELL, OI BLOOMFIELD, NEW JERSEY, ASSIGNOB TO WESTINGHOUSE LAMPCOMPANY, A CORPORATION OF PENNSYLVANIA.

ELECTRON-EMITTING MATERIAL.

lIo Drawing.

This invention relates to electron-emitting cathodes and moreparticularly to a cathode comprising a refractory metal body havingincorporated therein, material having big electron-emitting properties.7

In the present day manufacture of elec- 1 tron discharge devices, suchasrectifier, X- ray devices, radio receiving and transmitting tubes,etc., it is the very common prac- 10 tice to employ as the cathode, athermion i-;

cally active body composed of a refractory metal, such as tungsten,impregnated or mixed with a small percentage of thoria or thorium. Thepresent invention is especially concerned with an electron-emitting bodyof this nature.

One of the 0b 'ects of this invention is to provide a cathode of thethoriated type having substantially increased electron emission andwhich will maintain such high electron emissivity over a longcommercial. life.

Another object is to provide a method of producing such anelectron-emitting body.

A still further object is to produce a filamentary refractory metal bodyof increased strengt and toughness.

Other objects and advantages will hereinafter appear.

The present invention is based upon the discovery that when an oxide ofa rare earth metal of the cerium group is added in small proportions toa thoriated refractory metal body and the same subjected to the usualmanufacturing and activating processes, that such material imparts tothe body a very substantial increase in electron emission. Thisincreased electron emission, moreover, is maintained for a much longereriod than in the case of plain thoriated wire and the filaments towhich this rare earth oxide has been added are strongerand tougher thanordinary thoriated tungsten wire.

While thoriated tungsten is the preferred electron-emitting materialwith which the present invention is concerned, it is to be understoodthat the invention is not so lim-- ited but may be employed with thesame advantageous results to other refractory metal bodies, such asmolybdenum, havin other electron-emitting metals incorporate there- "in,such as zirconium, uranium, vanadium,

etc.

In the manufacture of thoriated tungsten filaments for electron emissionpurposes it Application filed February 24, 1927. Serial No. 170,744.

is the usual practice to incorporate up to 3% of thorium'oxide with thetungsten metal in powdered form and to fabricate the mixture into wireform by pressing, swaging, and drawing. The thorium oxide is converted,at least in part, to thorium metal, either during the fabricationprocess or subsequently and forms the active electron-emittingingredient of the filament.

I have found, however, that when an oxide of a metal of the cerium groupof rare earth metals is added to the thoriated-tungsten body in theproportion of aboutv.5 to 4% of the rare earth oxide to the thoriumoxide content that the electron emission obtained from the cathode isincreased very substantially. For instance, with a thoriated tungstenfilament containing 1% of thorium oxide and from .01 to .03% ceriumoxide, an increase in emission over plain thoriatedtungsten filaments offrom 10 to 40% was obtained. However, it was found that when thepercentage of cerium oxide was increased to 5% or more of the thoriumoxide content, it acted detrimentally rather than beneficially.

The action of the rare earth oxide in increasing electron emission isnot clearly understood but one possible explanation may be that thecerium either in oxide or metallic form, forms an alloy or mixture withthe thorium or thorium oxide and that such alloy or mixture has agreater efliciency' for transforming heat energy into electronicemission than does the plain thorium. I do not desire to be limited byany particular theory, however, since other plausible theoricsmight besuggested in explanation of the phenomena which I have observed.

Various methods may be employed for incor orating the cerium oxide orother oxide of t e cerium group of rare earth metals into the thoriatedtungsten body but the method I prefer to em loy is to prepare a solutionof thorium an cerium nitrates in such proportion as to produce, uponreduction, the 100 desired amount of thorium oxide and cerium oxide.This solution may then be intimately mixed with the proper pro ortion ofpreviously purified tungstic oxi e. .The proportions which I refer to emloy are 1% to 5 2% thorium oxide, with cerium oxide resent in theproportion of 1% to 3% o the thorium oxide content and the remaindertungsten. This mixture is then dried at .about 110 C. until free frommoisture in the usual manner. It is then crushed and sieved and thedried oxide fired in a muflle furnace at from 900 to 1000 for aboutthree hours and the contents removed and again sieved. 'During this heattreatment the cerium nitrate and thorium nitrate are converted to theoxide. After sieving, the fired material is placed .in a nickel boat andheated in a hydrogen furnace at about 1000 C. for a period of about twohours. The hydrogen effects a reduction of the tungsten oxide but doesnot reduce, to any extent, either the cerium or thorium oxide. Y I

It is essential that both the cerium and, thorium oxides be retained inthe oxide form since otherwise these metals would volatilize out of thetungsten during the subsequent heat treatment. The mixture is thenpressed into slugs or ingots and the ingots heated in a hydrogen furnaceat about 1200 for from one to two hours. This renders them sufiicientlycoherent to withstand the subsequent manipulations. Theformed and bakedingots "are then sintered in a treating bottle through which dryhydrogen as continuously flows and gradually heate up by pas sage ofelectric current therethrough to around 90% of the fusion current. Thesintered ingots are then swaged and drawn into wire form in accordancewith the usual practice and after completion of the wire drawing processthey may be activated by' any of the approved processes for activatingthoriated tun sten wire for electron emission purposes. n the case ofsmall sized wires activation may be accomplished by flashing thefilament in hydrogen for a short interval as set forth in copendingapplication Serial No. 110,047, filed May 19, 1926, Rich et al.,entitled Electron-emitting devices and method of preparation. In thecase of larger sized filaments it may be necessary to carbonize thefilament by flashing the same in a hydrocarbon gas in accordance withthe procedure set forth .in U. S. Patent 1,529,597,

' Langmuir, entitled Electron-emitting device and method of preparation.

" During this activation process or during the latter part of the Wiredrawing process the thorium oxide or a portion thereof and a portion ofthe cerium oxide appears to be reduced to the metal and apparently bysome coaction of the cerium and thorium the elec tron emission obtainedof the cathode is increased very substantially over that obtained fromthe plain thoriated-tungsten filaments. In addition to the largeincrease in initial emission, an increase in 'life, insome casesamounting to several hundred per cent has been obtained, such life beingbased on the number of hours the filament may be continuously operatedbefore the electron emission falls below a predetermined minimum.

this r This emission maintenance property of the present filament is ofextreme 1m ortance since considerable difficulty has con experienced-heretofore, due to early failure in emission of plainthoriated-tungsten cathodes.

It is obvious, of course, that many changes and modifications'may bemade in the process of producing the wire and various rare earth oxides,electron-emitting metals and refractory metal carriers may be employedwithout departing from the invention, but I do not desire to be limitedto the exact details shown and described.

What is claimed is: i

1. A thermionically active electrode comprising a refractory metal bodyhaving incorporated therewith a metal of good electron emissivity and asmall proportion of a metal of the cerium group of rare earth metals.

2. An electron-emitting body com rising a refractory metal containing asma proportion of a. metal of good electron emissivity and a metal ofthe cerium oup of rare earth metals, the proportion o the latter metalbeing small in comparison to the electron-emitting metal.

3. An electron-emitting body comprising a thoriated-tungsten bodycontaining a small proportion of a metal of the cerium group of rareearth metals.

4. A refractory metal body composed of tungsten containing a smallpercentage of thorium and cerium, said body having an electronemissivity at a given temperature substantially greater than plainthoriatedtungsten.

5. An electron-emitting body composed of a refractory metal containingfrom 1 to 3% of thorium or thorium oxide and from .005 to 12% of ceriumor cerium oxide.

6. A thoriated refractory metal body for electron emission purposescontaining cerium 8. A refractory metal body adapted to be sisting oftungsten containing from 1 to 3% of thorium oxide and from .5 to 4% ofceper centnof thorium oxide and cerium activated for electron emissionpurposes conrium oxide based on the thorium oxide content.

9. The method of increasing the electron emissivity of a thoriatedrefracto metal body which comprises adding tojsai body a smallpercentage of cerium oxide.

10. The method of making thermionically active electrodes which consistsin incorporating cerium oxide and thorium oxide into a'refractory metalbody and subsequently reducing a portion of the oxides to metallic form.

11. The method of producing filamentary bodies having high electronemissivity which 5 comprises incorporating into a tungsten body from 1to 3% of thorium oxide and from .5

to 4% of cerium oxide based on the thorium oxide content andsubsequently reducing, at

least in part, said oxides to metallic form.

In testimony whereof, I have hereunto 10 subscribed my name this 23rdday of February, 1927.

CHARLES V. IREDELL.

